Problem: Which of these numbers is prime? ${31,\ 49,\ 80,\ 81,\ 99}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 31 are 1 and 31. The factors of 49 are 1, 7, and 49. The factors of 80 are 1, 2, 4, 5, 8, 10, 16, 20, 40, and 80. The factors of 81 are 1, 3, 9, 27, and 81. The factors of 99 are 1, 3, 9, 11, 33, and 99. Thus, 31 is a prime number.